An audio transmitter (T) and a receiver (R) are hung vertically from two identical massless strings of length 8 m with their pivots well separated along the $$X$$ axis. They are pulled from the equilibrium position in opposite directions along the $$X$$ axis by a small angular amplitude $$\theta_0 = \cos^{-1}(0.9)$$ and released simultaneously. If the natural frequency of the transmitter is 660 Hz and the speed of sound in air is 330 m/s, the maximum variation in the frequency (in Hz) as measured by the receiver is ______.

(Take the acceleration due to gravity $$g = 10$$ m/s$$^2$$)